Windows only: Freeware application MP3 to iPod Audio Book Converter takes your regular MP3 audiobooks and converts them to the iPod (M4B) audiobook format, allowing you to take advantage of the features afforded to iPod audiobook files.

Namely, that means that your iPod will mark your last listening position so you can pick up where you left off (which you can more tediously do with MP3s) and let you adjust the playback speed. If you've got many MP3 audiobooks, or even podcasts, converting them to the M4B format—while less portable—should give you a nice added level of audiobook friendliness. If your files are already in M4A format rather than MP3, all you have to do is rename the extension to M4B.